In recent years, research and development are energetically made on electrophotographic photosensitive members making use of organic photoconductive materials (i.e., organic electrophotographic photosensitive members).
The electrophotographic photosensitive members are each basically constituted of a support and a photosensitive layer formed on the support. Under existing circumstances, however, various layers are often formed between the support and the photosensitive layer in order to, e.g., cover defects on the support surface, improve coating performance for the photosensitive layer, improve the adhesiveness between the support and the photosensitive layer, protect the photosensitive layer from electrical breakdown, improve chargeability, and improve the performance of blocking the injection of electric charges from the support to the photosensitive layer. Thus, the layers to be formed between the support and the photosensitive layer are required to have many functions such as covering properties, adhesiveness, mechanical strength, conductivity, electrical barrier properties, and so forth.
The layers to be provided between the support and the photosensitive layer are conventionally known to include the following types.    (i) A resin layer containing no conductive material.    (ii) A resin layer containing a conductive material.    (iii) A multi-layer formed by superposing the type-(i) layer on the type-(ii) layer.
The type-(i) layer contains no conductive material, and hence the layer has a high resistivity. Moreover, in order to cover defects on the support surface having not been subjected to surface smoothing treatment, it must be formed in a large thickness (layer thickness).
However, if such a type-(i) layer having a high resistivity is formed in a large layer thickness, a problem may arise such that it brings about a high residual potential at the initial stage and during repeated use.
Accordingly, in order for the type-(i) layer to be put into practical use, it is necessary to lessen defects on the support surface and also to form the layer in a small layer thickness.
On the other hand, the type-(ii) layer is a layer in which a conductive material such as conductive particles are dispersed in a resin, and the layer can be made to have a low resistivity. Hence, the layer may be formed in a large layer thickness so as to cover defects on the surface of a conductive support or a non-conductive support (such as a support made of a resin).
However, where the type-(ii) layer is formed in a large layer thickness, the layer must be endowed with a sufficient conductivity, compared with the type-(i) layer to be formed in a small thickness, and hence the type-(ii) layer results in a layer having a low volume resistivity. Hence, in order to block the injection of electric charges from the support and the type-(ii) layer into the photosensitive layer, which is causative of image defects, under environmental conditions ranging broadly from low temperature and low humidity to high temperature and high humidity, it is preferable that a layer having electrical barrier properties is additionally provided between the type-(ii) layer and the photosensitive layer. Such a layer having electrical barrier properties is a resin layer containing no conductive material, such as the type-(i) layer.
That is, the layer to be provided between the support and the photosensitive layer may preferably have the type-(iii) constitution in which the type-(i) layer and the type-(ii) layer are superimposed one on another.
The type-(iii) constitution requires formation of a plurality of layers, and hence requires steps in a correspondingly larger number. However, it has such an advantage that the tolerance for defects on the support surface can be of a wide range, and hence the tolerance for the use of the support can be of a vastly wide range, promising the achievement of improvement in productivity.
In general, the type-(ii) layer is called a conductive layer and the type-(i) layer is called an intermediate layer (a subbing layer or a barrier layer).
An aluminum pipe produced by a production process having an extrusion step and a drawing step and an aluminum pipe produced by a production process having an extrusion step and an ironing step are used as supports for electrophotographic photosensitive members, which can achieve a good dimensional precision and surface smoothness as non-cut pipes without requiring surface cutting and besides are advantageous in view of cost as well. However, burr-like protruding defects tend to occur on the surfaces of these aluminum non-cut pipes. Thus, from the viewpoint of covering surface defects of such supports, too, the type-(iii) constitution is preferred.
As the conductive material used in the conductive layer, it includes various metals, metal oxides and conductive polymers. In particular, tin oxide (hereinafter also “SnO2”) having powder resistivity usually in the range of from 104 to 106 Ωcm is preferred as having superior resistivity characteristics. A conductive material is also available whose powder resistivity is reduced to 1/1,000 to 1/100,000 by mixing (or doping), when the SnO2 conductive material is produced, a compound of a metal having a valence different from tin, such as antimony oxide, or a non-metallic element. An oxygen deficient SnO2 conductive material is also available in which the resistivity of SnO2 has been made to be as small as that of antimony doped materials without adding constituent elements and in a non-doped state.
As prior art relating to oxygen deficient SnO2, Japanese Patent Application Laid-open No. H07-295245 for example discloses a technique making use of oxygen deficient SnO2 in a conductive layer. Japanese Patent Application Laid-open No. H06-208238 also discloses a technique in which barium sulfate particles are coated with oxygen deficient SnO2 so that dispersibility can be further improved than that in a case in which only SnO2 is used. Japanese Patent Application Laid-open No. H10-186702 still also discloses a technique in which barium sulfate particles are used in order to improve dispersibility, the particles being coated with titanium oxide (TiO2) in order to improve whiteness and further coated with SnO2 in order to provide conductivity. This Japanese Patent Application Laid-open No. H10-186702, however, does not disclose any embodiments of the oxygen deficient SnO2.
In recent years, an electrophotographic apparatus has become widely used employing a contact charging system in which a voltage is applied to a charging member provided in contact with an electrophotographic photosensitive member (i.e., a contact charging member), to charge the electrophotographic photosensitive member. In particular, a system is prevalent in which a roller-shaped contact charging member (a charging roller) is brought into contact with the surface of an electrophotographic photosensitive member, and a voltage generated by superimposing an alternating-current voltage on a direct-current voltage is applied thereto to charge the electrophotographic photosensitive member (an AC/DC contact charging system), or in which only a direct-current voltage is applied to the charging member to charge the electrophotographic photosensitive member (a DC contact charging system).
In the AC/DC contact charging system, there are disadvantages, e.g., such that a direct-current power source and an alternating-current power source are required to bring about a rise in cost of the electrophotographic apparatus itself and that the size of electrophotographic apparatus becomes enlarged as compared with the case of the DC contact charging system. In addition, there is such a disadvantage that alternating current consumed in a large quantity causes deterioration in durability of the contact charging member and electrophotographic apparatus.
Accordingly, taking into account cost reduction, compactness and high durability, the DC contact charging system can be said to be more preferred.
However, electrophotographic apparatus employing the DC contact charging system tend to become inferior in the uniformity of the surface potential of the electrophotographic photosensitive member at the time of charging (i.e., charging uniformity) to electrophotographic apparatus employing the AC/DC contact charging system. Accordingly, faulty images caused by non-uniform charging and appearing in non-uniform lines in the lengthwise direction (the direction perpendicular to the peripheral direction) of the electrophotographic photosensitive member (hereinafter also “charging lines”) are apt to bring about a problem in halftone images or the like.
In regard to such a problem, a proposal for improvement has been made in respect of the charging member. Specifically, as a measure for improving charging uniformity, studies are made on how to make the resistance distribution of the charging member uniform and improve the surface properties of the charging member.
As to the former, measures are available in which, e.g., the dispersion of a conductive material in a surface layer (outermost layer) of the charging member is improved, a resin having a relatively low volume resistivity is used in a binding material of a surface layer of the contact charging member, and the layer thickness of each layer constituting the contact charging member is adjusted to be uniform.
As to the latter, measures are available in which, e.g., a leveling agent is added to a surface layer of the charging member, and an elastic layer of the charging member is improved in surface properties.
In the case where only a direct-current voltage is applied to the charging member to charge the electrophotographic photosensitive member, Japanese Patent Application Laid-open No. H05-341620 proposes a technique in which the surface roughness of the charging member is made to be 5 μm or less to achieve the charging uniformity.
Japanese Patent Application Laid-open No. H08-286468 proposes a technique in which that the ten-point average roughness Rz jis (JIS B 0601) of the charging member surface is made to be 20 μm or less in order for the charging uniformity to be secured to provide good images.
According to the above proposals, the improvement of initial-stage charging uniformity can be substantially achieved, but under existing circumstances, is insufficient in respect of stabilizing the charging uniformity. More specifically, as a result of long-term service, contaminant such as developer dust or paper dust adheres to the surface of the charging member. In that case, where they come to adhere partially non-uniformly or adhere in a large quantity, they may lower the charging uniformity.
For the subject of stabilizing the charging uniformity during long-term service, proposals are made in which the surface roughness is further adjusted to make improvement. For example, in Japanese Patent Applications Laid-open No. 2004-061640 and No. 2004-309911, a technique is disclosed in which the surface roughness of the charging member is controlled to secure the charging uniformity. Also, in Japanese Patent Applications Laid-open No. 2004-038056, a technique is disclosed in which the surface roughness of the charging member and the coefficient of surface friction of the electrophotographic photosensitive member are controlled to secure the charging uniformity.
In general, it is known that contaminant can be better kept from adhering to the surface of the charging member as a result of long-term service as the charging member has a smaller surface roughness. Also, if it has too large surface roughness, faulty images such as spots may come about because of faulty charging due to the surface shape of the charging member. From these viewpoints, it is more preferable for the charging member to have a smaller surface roughness.
Electrophotographic apparatus are more highly demanded to achieve higher speed and higher image quality. In particular, as images have come to be reproduced in colors (in full color), halftone images and solid images have come to be often reproduced, and such a demand for higher image quality increases steadily year by year.
For example, importance is attached to the uniformity of density and tint in reproduced images on each sheet and the stability in continuous image reproduction, and tolerance therefore have become remarkably severer as compared with that in black-and-white printers and black-and-white copying machines. In particular, in electrophotographic apparatus employing the DC contact charging system, records in one cycle of electrophotographic processing tends to appear as charge potential non-uniformity of the electrophotographic photosensitive member, which may cause ghost due to records of exposure (image exposure) and charge memory due to transfer (transfer memory). Then, as a result, density non-uniformity may come about in reproduced images.
Accordingly, in usual cases, a measure is applied in which a charge elimination (de-charging) means such as a pre-exposure means is provided on the downstream side of a transfer means and on the upstream side of a charging means so as to eliminate records in one cycle of electrophotographic processing and eliminate the non-uniformity of surface potential of the electrophotographic photosensitive member.